The present invention relates to a drift eliminator, sometimes referred to as a mist eliminator, a critical element in both direct and indirect counter-flow and cross-flow cooling towers, spray filled towers, and evaporative condensers, all of which are hereinafter categorically referred to as "tower". More particularly, the drift eliminator of the present invention has a structure that improves drainage, elevates the drift point, and reduces air pressure drop and therefore power requirements in the equipment in which it is used, resulting in enhanced thermal efficiency.
A drift eliminator has the function of stripping entrained water droplets or mist from the gas (air, typically) stream passing out of the tower. Without drift eliminators, evaporative cooling equipment would be impractical because a majority of the circulating water would be blown right out of the top of the tower. Further, drift is very objectionable because of excess loss of circulating water, loss of water treatment chemicals, and wetting of the space surrounding the tower with its ensuing hazard potentials such as ice formation, staining of surrounding cars and buildings, and spread of bacteria.
A drift eliminator is normally comprised of a plurality of channeling elements having a shape, curved or otherwise, which causes one or more changes in the air direction passing through the channel. The channel shape is designed such that air will pass through the channels with a minimum pressure drop, but the air direction change is too sharp for water droplets entrained in the air stream to pass. The momentum of the drop will cause it to impinge on the surface of the drift eliminator as it changes direction, and the action of gravity will cause the droplets to flow over the surface of the eliminator and fall back into the cooling, tower. Water droplets that make it past the curvature will normally continue to be carried along with the exiting air. This escaping water spray known as "drift" or "carry-over" emanates from the equipment. While this is the primary mode of drift formation, other modes exist. The second most common mode of drift formation occurs when upward air pressure and fluid shear forces reverse the downward gravity force on the water impinged on the drift eliminator surface and push the accumulated water film to the top edge of the eliminator surface. The water film on the top edge of the eliminators coalesces to large drops that are picked up by the passing air and ejected from the tower. Large water drop drift is most commonly encountered when eliminators have become flooded, that is, their impingement surface has been coated with a thick water film.
"Drift rate" relates to the amount of water spray that is carried out of the tower with the air. It is quantitatively measurable and is commonly expressed as a percentage of the circulating water flow in a tower. Drift rates are usually very small, less than 0.01%, and drift eliminators are selected by their drift rate performance which varies with tower application and characteristics that differ based on their manufacturer. The air velocity at which drift eliminators fail to function by exceeding the specified drift rate, thereby passing excessive drift out of the tower, is known as the "drift point" or "spit point". In a tower, many factors influence both the drift point and the mode of drift formation. These factors include the geometry of the eliminator, the proximity of the eliminator to the tower's water distribution system, the circulating water flow inside the tower, the predisposition of the air pattern approaching the eliminator, water quality, fouling coatings on the eliminator, and the operating time history of the tower. However, for a given eliminator geometry and tower geometry, the controlling factors on the drift point are circulating water flow and air velocity.
FIG. 11 schematically graphs the relationship between circulating water flow and air velocity inside a tower and its effect on the drift point. Circulating water rate is shown as the ordinate, and air velocity is the abscissa. A drift point curve 100 separates a region 102 of acceptable drift rate, which is below the curve, from a region 104 of excessive drift, which is above the curve. The geometry of the eliminator and the geometry of the tower will act to shift the drift point curve generally upward or downward. For modern, high-efficiency drift eliminators, the drift point corresponds to an air velocity typically in the range of 500 to 800 feet per minute (fpm) in the passageway where the drift eliminator is mounted in the tower at circulating water flow rates over the wet deck fill of 3 to 20 gallons per minute per square foot (gpm/ft.sup.2) of tower plan area. It should also be noted that as the circulating water rate increases, the eliminator tends toward flooding and large droplet drift formation.
Some equipment designs or common maintenance problems predispose drift eliminators to operate at lower drift points than would be desired for the tower's intended purpose. Examples of the aforementioned are towers with high circulating water rates, towers with eliminators directly above but in contact with the circulating water spray system, or towers with poorly maintained water distribution nozzles that force water spray directly into the drift eliminators because these conditions tend to flood eliminators. When eliminators are flooded, their consequent drift point is reduced and the tower's air velocity must be reduced to prevent drift. The result is a loss in overall thermal capability.
It is typical on large counter-flow towers, particularly the field-erected type, to have a substantial plenum space between the spray system and the drift eliminators, moderate water flows, moderate air flows, and regular maintenance attention, all of which create a more ideal eliminator application environment. In contrast, factory assembled towers (condensers) have shipping constraints that require a very compact tower, and the drift eliminators are assembled directly on top of the spray system. Further, the broad demand for energy efficient, compact, factory-assembled towers dictates the use of eliminators in the adverse environments of high circulating water flows (8-20 gpm/ft.sup.2 inside the tower), high air velocity (600-800 fpm), and continual operation with minimal spray system maintenance. The present invention particularly, but not exclusively, targets drift eliminators for these demanding conditions associated with factory-assembled towers.
The current state of the art for counter-flow drift eliminators can generally be divided between parallel blade eliminators and cellular eliminators. Most induced draft towers use cellular eliminators, while most forced draft towers use parallel blade eliminators, but there is no specific requirement for either. Parallel blade eliminators typically have a plurality of parallel curved blades made into sections that can be handled by maintenance personnel. The blades are separated by discrete spacers, which may be separate items or integrally formed in the blade, or the blades may be spaced apart by formed end caps which retain the ends of the blades in a defined spaced relationship.
The most common eliminator used in induced draft counter-flow towers is a cellular drift eliminator. When viewed from the top or bottom, a cellular drift eliminator appears as a plurality of parallel, curved tubes and is distinguishable from an eliminator made of parallel blades. The cellular drift eliminator may be comprised of a plurality of parallel curved surfaces that create the impingement surfaces for water droplets. Between the curved surfaces, a spacing element forms small cells, as in the present invention. Cellular eliminators have also been fabricated from mating corrugated curved blades. The distinctive features of cellular eliminators are that the tubular design adds strength to the eliminator assembly and creates further traps for water migration and droplet impingement. Because of the design, cellular drift eliminators have a higher drift point than parallel blade eliminators, but at a slightly higher pressure drop. Cellular drift eliminators are normally flat on the top and bottom. The strong, flat structure of the cellular eliminator allows wider support spans, smaller supports, and/or more top-side structural support so maintenance personnel can stand on or walk across the top of the eliminator.
The prior art relating to parallel blade eliminators is crowded with improvements on the geometry of curvature, trailing edges, leading edges, and construction. The most relevant prior art to the current invention of which the inventors are currently aware are U.S. Pat. Nos. 4,601,731 and 5,464,459, which disclose drift eliminators having a plurality of parallel blades, where the lower edge of each blade is cut with drainage teeth in a saw tooth pattern. U.S. Pat. No. 4,601,731 discloses that this pattern provides enhanced water drainage and reduced pressure drop. The present invention is distinctive not only in geometry but also in mechanism of operation from the prior art.
The present invention is an improved cellular drift eliminator comprising blades and spacer elements that create the cells of the cellular drift eliminator. Only the lower edges of the spacer elements have cut out portions. The cut out portions concentrate and promote water drainage from the spacer portion of the eliminator assembly toward the parallel impingement blades. Because the invention has improved drainage of the spacer section, it remains drier, raising the useful drift point of the eliminator by approximately 10%, particularly at conditions that would flood a comparable eliminator without the improvements.
The effectiveness of the present invention to raise the drift point, especially at flooding conditions, is graphically illustrated in the drift rate versus circulating water flow curve of FIG. 11. The drift point curve 100 is representative of a common cellular drift eliminator. Drift point curve 106 is representative of a parallel blade drift eliminator, showing its reduced capability as indicated by a lower drift curve, corresponding to a greater area of unacceptable drift above the curve. Drift point curve 108 is representative of the improved cellular drift eliminator of the present invention.
The advantage of a cellular drift eliminator's increased impingement surface is, however, detrimental to low air pressure drop. In a cellular eliminator, a water film within the interior passages of a cell reduces the effective area for air to pass, resulting in increased pressure drop. Another advantage of the invention is that it reduces the air pressure drop of a cellular eliminator to correspond to the air pressure drop typically associated with a parallel blade eliminator. The cut out portions in the spacer elements allow a wider and more open region for air to enter into the eliminator structure. Water drainage is concentrated toward the widely spaced parallel blade surfaces rather than from simultaneously all around the bottom edges of the blades and spacers which lie in the same plane, keeping the interior of the cells drier than if the cutout portions were not present. By keeping the interior of the cells drier, air has the maximum area of passage at minimum pressure drop.
Lower air pressure drop and higher drift points allow the present invention to enhance both thermal efficiency and thermal capability in a tower. Further, the improved cellular eliminator retains its structural integrity and provides a flat mounting surface inside a tower. Unlike the invention of aforementioned parallel blade patent, U.S. Pat. No. 4,601,731, the present invention is structurally superior, because its weight is supported on the flat bottom surfaces of the eliminator blades rather than concentrated only on the drainage points, and it does not need any special or elaborate support modifications in the tower. Retaining the structural integrity of the bottom surface is particularly important because the eliminators are often used as a working platform by maintenance personnel and because eliminators often need to be replaced in the field without the added difficulty of tower structural modifications.
The present invention was a surprising discovery in view of laboratory testing of parallel blade eliminators with saw tooth drainage points similar to the prior art compared to parallel blade eliminators without such saw tooth drainage points used in the same tower apparatus of the test. Such laboratory testing has not supported all of the benefits claimed for including saw tooth drainage points. Thus, the testing demonstrated that there was no significant difference in performance using the parallel blade eliminators with or without saw tooth drainage points. Adding saw tooth drainage points did not measurably reduce the pressure drop, and the drift rate increased slightly, possibly due to a reduction in impingement surface.
The drift eliminator improvements of the present invention are intended to and do dry the eliminator and raise the drift point, especially at high circulating water flows. Because water films are relatively thin, and drainage drops on cellular drift eliminators are relatively small, significant pressure drop reductions were not expected from drainage improvements, especially in view of the test results reported above concerning the use of saw tooth drainage points on parallel blade eliminators. Surprisingly, the invention exceeded expectations in reducing the eliminator pressure drop, and therefore, improving the thermal efficiency of the apparatus by 1-3%. Both higher drift points and improved thermal efficiency were achieved in the tower apparatus with the present invention. Improving thermal efficiency to this apparently small extent is actually quite significant. With this type of heat exchange equipment, it is difficult to improve thermal efficiency at all. Many modifications that have been made throughout the years actually reduce the thermal efficiency, in view of the complicated interaction of all components with the variables of water flow and air flow in any given system. Since these types of heat exchange systems are already quite efficient, even apparently small improvements to thermal efficiency are quite significant in meeting legislated energy efficiency mandates as well as in benefiting the tower user with reduced energy costs over the lifetime of the equipment.
Such drift eliminator enhancements of the present invention can be applied to both induced draft and forced draft counter-flow equipment. Further, drift eliminators may be used in indirect towers (closed circuit coolers) and evaporative condensers with equal benefit. Although the liquid is usually water and the as is usually air, the drift eliminators can be used with systems designed for other liquids and other gases.